You need to buy a Motorola MX4200 GPS receiver and level converter box.
Easiest way to get the exact order info may be to contact ISI. The
receiver doesn't cost much ($350?), but you need to sling the antenna
where it can see the horizon in most directions, like on the roof, and
run wires to the router.

The device we decided on picks up GPS info and sits on the network and
provides the time data in NTP (version 2 and 3) as well as time code,
1pps, and frequency reference signals. Its a Tymserve 2100 GPS from Datum-Bancomm, and
lists for about $6,000.